Incomplete implementation of assignments in Compile.hs

[ryzhyk]

We currently don't have a way to compile arbitrary l-values to Rust, e.g.,

Constructor{var x, var y} = z

is ok, but

Constructor{x, y} = z

where x and y are previously introduced variables is illegal.

Either implement the logic to support this translation (e.g., by assigning to intermediate variables) or modify validation logic to disallow such programs